For the many fans who fell in love with Brazil’s influential psychedelic group Os Mutantes during the three decades since they stopped playing together, it’s a dream come true that this cult favorite is having a reunion tour. And it’s a bit of a freak-out because they’re coming to Chicago for the first time to play at next week’s Pitchfork Festival.

But what seals the deal is the fact that Os Mutantes (pronounced “ohs moo-TAN-chees”; it means “The Mutants” in Portuguese) founder Sérgio Dias will rock the giant, gold-plated, custom-made guitar—with distortion and delay effects built in—that his elder brother Cláudio handcrafted in the late ’60s. (Cláudio, though never in the band as a musician, engineered many of Os Mutantes’ custom instruments and effects.) “We’re gonna blow your head off! Get ready, man,” Dias laughs.

Once universally praised for its Technicolor pop-art albums—which combined literary references like Don Quixote with samba and psych-rock—the group had a May show at London’s Barbican that brought brothers Sérgio Dias and Arnaldo Baptista together onstage for the first time in 33 years.

Like the young Beatles tuning into Radio Luxembourg, the teenage brothers first heard rock via shortwave radio in1960s Brazil and formed a band with Arnaldo’s girlfriend, singer Rita Lee. The trio went on to become the backbone of the kooky, intellectual and artistic movement Tropicália, which rebelled against Brazil’s cultural and political stagnation. Mutantes made its way to Paris, where the band cut an English-language album. But tension between the brothers, heavy acid use, and Arnaldo and Rita’s breakup led to reshuffling in 1972, a move toward prog-rock in 1974 and the band’s eventual demise in 1978. Lee went on to Brazilian mainstream stardom and Mutantes faded into obscurity—Dias played jazz and new age, and Arnaldo spent some time in a mental-health facility.

But the band’s inimitable sound developed a cult following: Kurt Cobain invited the group to tour with Nirvana (even though it hadn’t existed for 15 years), members of Fugazi collected paintings by former Mutantes members, and Beck named his 1998 Mutationsalbum in their honor. Mutantes’ earlier albums became available on CD in the late ’90s—David Byrne’s Luaka Bop label issued a compilation of early material, and this year the group’s catalog is being reissued worldwide by Universal.

In one sense, the Mutantes comeback is right on time. “God, can you imagine if we surfaced in the ’80s?” asks Dias, who’s stayed hip producing underground acts in his São Paulo studio. “This would be a total disaster. It didn’t have anything to do with us. Now, I think the musical scene is more toward what we did and what we do. I think it was a very healthy way to come back and come alive again.”

It’s unclear who sowed the seeds of the reunion, but Dias’s account points to an ambitious festival curator in England. “I started to receive mail saying that we were going to play in London, and then suddenly we started to talk about it because we didn’t know anything about it,” he says. “It was really amazing. Suddenly I was talking to Dinho, the drummer, and he said, ‘If you want to, we can try. I can try to play.’ I never heard him say anything like that before.” Lee won’t be joining the band, citing family commitments. “She will always be a Mutante—she is more than welcome to come over,” Dias says. Brazilian pop singer Zélia Duncan will sing Lee’s parts on the U.S. tour.

The brothers and drummer Dinho (who joined the group in 1970) will play as part of a ten-piece band to reproduce the multiple layers of Mutantes’ recordings. “There’s a lot of things to be covered,” Dias says. “We don’t want to use a damn computer and play with a click. Everything is played. There’s flutes, there’s recorders.”

Mutantes will also mix in some English-language versions of songs to court the American audience. In addition to Pitchfork, they’ll play venues such as the Fillmore in San Francisco, where Dias saw psych blues band Ten Years After at age 17. “It’s outrageous—we’ve never been in the States and suddenly I’m going to be playing at the Fillmore,” he says. “It’s something that can make someone very humble and humbly proud of the right decisions that we took when we were at such an early age.”

Os Mutantes headlines the Pitchfork Music Festival Sunday 30.

Brazil's Os Mutantes, the psychedelic pop group that inspired Nirvana, Fugazi and Beck, plays its first Chicago gig at Pitchfork. By John Dugan Photograph by Nino Andrés